The electrical impedance of a circuit element is the measure of the element's opposition to the passage of a current when a voltage is applied. In quantitative terms, the electrical impedance is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase (unlike resistance, which only has magnitude). When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle. The concept of impedance is introduced in AC circuits because there are other mechanisms impeding the flow of current besides the normal resistance of DC circuits, such as inductance and capacitance. Various electrical circuits may be formed from one or more elements providing impedance, such as resistors, capacitors, and inductors. Depending on the impedance that is introduced to a particular circuit, a different result and/or function can be achieved.